Fuel injection into the cylinders of an internal combustion engine is most commonly achieved using either a unit injector system or a fuel distribution type system. In the unit injector system, fuel is pumped from a source by way of a low pressure rotary pump or gear pump to high pressure pumps, known as unit injectors, associated with corresponding engine cylinders for increasing the fuel pressure while providing a finely atomized fuel spray into the combustion chamber. The fuel distribution type system, on the other hand, supplies high pressure fuel to injectors which do not pump the fuel but only direct and atomize the fuel spray into the combustion chamber.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate.
One method of reducing the initial volume of fuel injected during each injection event is to reduce the pressure of the fuel delivered to the fuel injector nozzle assemblies during the initial stage of injection. As a result, various devices have been developed to control or shape the rate of fuel delivery during the initial phase of fuel injection so as to reduce the fuel pressure delivered to the nozzle assemblies. For example, U.S. Pat. Nos. 3,669,360, 3,718,283, 3,747,857, 4,811,715, 3,817,456, 4,258,883, 4,889,288, 5,020,500 and 5,029,568 disclose devices associated with each injector nozzle assembly for creating an initial period of restricted fuel flow and a subsequent period of substantially unrestricted fuel flow through the nozzle orifice into the combustion chamber. However, these rate control devices require modifications to each of the fuel injector assemblies in a multi-injector system thus adding costs and complexity to the injection system.
Other fuel systems include rate shaping devices positioned upstream of the injector for controlling the initial volume of injected fuel. For example, U.S. Pat. No. 4,993,926 to Cavanagh discloses a fuel pumping apparatus capable of rate shaping which may be fluidically connected to a plurality of injectors via a distributor member. The fuel pump includes a piston having a passage formed therein for connecting a chamber to an annular groove for spilling fuel during an initial portion of an injection event. The piston includes a land which blocks the spill of fuel after the initial injection stage to permit the entirety of the fuel to be injected into the engine cylinder. However, the rate shaping pump delivers injection fuel directly to each injector during a pump stroke of the piston and thus the injection pressure is dependent on engine speed. As a result, although systems of this type can achieve the necessary pressures and injection accuracy under some engine conditions when provided with appropriate design and controls, such systems can not be relied upon to provide the desired performance objectives, such as very high injection pressures, over the long term especially at low engine speeds.
U.S. Pat. No. 4,838,232 to Wich discloses a fuel delivery control system including an injection rate control device positioned upstream of a fuel injector for creating an initial injection followed by a main injection. The control system includes a supply line of a specific length extending between a positive displacement pump and an injector assembly to create a hydraulic delay between initial and main injection events. The length of the supply line is chosen to create to a predetermined desired hydraulic delay corresponding to an ignition delay of the engine. However, the critical length of the supply line or passage extends between a fuel pump and an injector having a fuel control valve. Therefore, like the fuel system disclosed in Cavanagh discussed hereinabove, such a system can not be relied upon to provide the desired performance over the long term and especially at low engine speeds. Moreover, the Wich delivery control system creates a fixed rate shape or delay corresponding to the length of the supply line and therefore does not permit the rate of fuel flow to be shaped or varied during operation of an engine.
U.S. Pat. Nos. 4,711,209 and 5,054,445 to Henkel and Henkel et al., respectively, both disclose fuel injection systems including parallel fuel supply lines for creating pre-injection and main injection events. The fuel supply lines are designed with relative lengths such that the difference in lengths create different pressure wave traveling times and thus the desired delay between the pre-injection and main injection events.
Commonly assigned U.S. patent application Ser. No. 08/362,449 filed Jan. 6, 1995, discloses various rate shaping devices for use with an accumulator pump type system which effectively shape the rate of fuel injection by controlling the length of the fuel transfer passage connecting the accumulator to an injection control valve. These devices have been found to effectively slow down the rate of fuel injection during the initial portion of an injection event while subsequently increasing the rate of injection to rapidly achieve a high injection pressure.
Although the systems discussed hereinabove create different stages of injection, further improvement is desirable.